Java Swing - 如何保持JFrame到特定的宽度和高度

import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JPanel {
  private static final int PREF_W = 400;
  private static final int PREF_H = 300;

  @Override
  public Dimension getPreferredSize() {
    return new Dimension(PREF_W, PREF_H);
  }

  private static void createAndShowGui() {
    Main myPanel = new Main();
    JFrame frame = new JFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.getContentPane().add(myPanel);
    frame.setResizable(false);
    frame.pack();
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    createAndShowGui();
  }
}